Adherent electroplating on titanium



Unit dv Sims Pa fi fO -.-.2, 946; a. g LADHERENEELECTROPIJATINGDN .twiuiamr. Foisel, levela1id, n char! Eumers 'hagrin i allsgohio, assignors to'Cleveland Pneumatic lndustriesglnc ia-icorporatiomof Ohio NoDrawing. '.rueauunais;19ss;ser; No. 517,654

, 2,946,728 Patented July 26, .1960

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' agents wand. plat-ingbaths/employedin accordance with ourinvention may-.be .ine tpensive-and-easi'ly:compounded. lEurthermore, .the use. of. theselreagents andvbaths-inac- -.cordance with. our discoveries is --relatively.-simple, easy .and..str aight forward, :no. complicated, o'r--.diflicult controls .being necessary," and no 1 special operatofis .tskills being required. for. consistently satisfactory results.-

..There are several.importanuandsurprising discoveries we shave. .rnade whichprovide .the basisfomthe, present invention :inLboth its :method and article aspects. 1 in par'ticular, we foundthatfilms havingzunique and valuable 4 properties .canube. deposited on titanium and. .titanium -alloy surfaces whenthese .surfacesare contacted with-.cer-

' ;.t-ain solutions. containing.chromicvand. hydrofluoric. acids.

ponderant portions. Titaniumhas ahigh. strength-weight ratiotand,accordingly,'is. of special-interestin-the aircraft,

industry. However, titanium cannot be used for any purpose wherefthere is. rubbing.contactbetweentwo bodies .ofthis metal. -Y'Ihereiis. a markedtendency for titanium su rfaces-\to seize when they .arerubbed togetherso that .ordi11ar.y boltsarid .nuts .andjlandinggear parts cannot .be. made .ofititanium ,and .its alloys with lheexpectation of. satisfactory .service .even 'for the .'shortest,.periods of :tir'ne.

fIt' has been suggested previously" by others that .this diificulty .couldlbe eliminatedor substantially minimized iby providingicoatings of 1 harder metals. such as nickel .or

lchro'mium on titanium or titanium. a'lloybodies, "How- .ever, tofthe'best of-jourknowledg'e, no one has heretoffor'elbeenable to provide coatingsof this type which would bearupunder the ordinary conditions ofJuse. The problem is.to ejstablish a suflic'ientdegree of,,,adhesion between .Qthe. coatings andthe titanium ibase article; .andrpicogni'zing this, others skilled in the art'have madelrnany serious ;elfQrts along anurnber .of different lines to. -im-.

prove adhesion olf thesecoatin'gs totitanium articles.

. Direct the apparently promising efforts to solvethis pr'oblem .was directed along the mechanical. line, in that, 'it was attempted toestablish a keyed-type of .bond between the coating and the base a'rticle'. "Thusthetitanium'body was deeply'etched prior to electroplatingto providethe pits and grooves necessary to theestablishment of this type of attachment. It hasbeenfound, however, that in practice; this procedure cannot'be relied upon to produce consistently adherentcoatings; and it is, therefore, generally I regarded as inadequate and unsuccessful.

The conventional-=electroplatingprocedures oftheart developed. in .connection "with the plating of votherrmet-als havebeen attempted with titanium and its alloys without success. Anyffilms, coatings orplateswhich areithus -established -on ibesurface -of.the titanium bodyaare..not

sufficiently adherent to the base metal for any practical u'se'ofthe resulting article. By making the'present'inventionjwe have provide'd the "artfforthe f'first time, with a "completely satisfactory Esolution-to this problem. Thus,not only:canfilms,jcoatings or-plates be provided' on-"titanium 'articles'zto produce an object ofj practical utility"un der a wide variety of conditions where i'titanium was previously unsuitable, "butalso resultcan be obtained without incurring any "offsetting disadvantages'economic 'or"otherwise. 'lihere- We also :foundth at nickel. can .be electroplatedsdirectly onl'these films to produce a composite plate-which-adheres to the titaniumlbase metal far more strongly than anyother plate' heretofore known. .Still further,-we have found that,,by .heattreating thekesultingcomposite plate undercertain time-arid temperature conditions, the ad- 7 herence ofthe plate to -the base article was improved,

"apparently 'through'the establishment of an. alloy bond so that the resulting article iscapable.ofwithstanding severe mechariicallstress. We consider these discoveries surprising and unpredictable becausethe prior -art did not suggest thatsuch results could be obtainedandbe- 'causeithere, is noprece dent'for pursuing the courses which resulted in these discoveries and in the integration -of theminto' thepr'esentinvention.

:Des'cribe'd in abroadgeneral way, the method-ofour jjpres'e'nt .invention comprises the .steps vof limmersing a "surface dfia'n article of titanium or titanium alloyin-an fagueousacidlsolution of a chromium ,compoundwhich "contains trivalentichromium and fluoride.ionsandremovingi'thefarticle from'contact with the said solution when a'coating has ibeemformed on the saidarticle. 'Aslin dicated above, inqthe'production or an adherent composite coating, (this method includes the aidditionallisteps of electroplating,nickelon the said .coating and .thenlheat- :ing f'th'e jresultin'gfarticle and thereby forming a strong alloy bond"'between"the composite coatingand thearticle.

, Plates or deposits 'of'chrornfium or otherrsuita-ble metals *-.may"'readilytbe established upon the resulting composite wi de variety'of 'surfacecharacteri'stics.

Those skilled in the plating art will'reGQgniZeihat our ."coafting in subsequent operationsto produce articles are 45 f prese nt*;process:embodies certain entirelyi'new features 'and phenomenain' the chemistry and electrochemistr y of plating operations andnovelrea'ctions of c'hromiumicom- '--pounds. Thus, tolthebest of .ourknowledge'it has "never' beforebeenl'known thatfilms offthisgtype could "be "deposited ion ".a surfacefof another-metal from an "acidified aqueousjsolutioniof a chromium compound by "the technique of iminersioniplating. In'the course of '-'the reactions lea'ding' to this result, ,hydrogenfis evolved; and, at the same time, hexavalent chromium is reduced "through" the 'trivalent and possibly through "the divalent form and is deposited on the"titeanium"' baseIinetal as a ffi-lm' or i in "a film. We .are inclined .to lbelieve' that' "this filrn consists-"of metallic chromium, but because or the fextremef thinness of it, we .have'not been able tofp'rove this conclusively. I "Neverthelessf'this ;-reductipn gof chro- =-mmm compounds by titanium isentirely"unigue to the fbest 'offour :knowledge, Also, the deposition jof jsuc'h 'films ontitanium was neverheretoforeknowm'sofar'as we fknow. Furthermore, the utilization of a film" of this type on a titanium i or titanium "alloy farticlefas a base for 'subsequent-adherent'plates' ofnickel is entirelyinew and, in ia'dditiomrepresents, a'sindicated'above, aimost unexpected-.characteristicof chromium. n

We havef'found in pursuing the i'afores'aid discoveries and "in the developem'ent "of the process of the"presen't invention and of its preferred form that there are certain conditions which are critical to consistently satisfactory results. No films, coatings or plates of this type can be established on a titanium or titanium alloy surface by immersion deposition from freshly prepared solutions of chromic and hydrofluoric acids without some activating means through which some trivalent chromium is provided in solution. This means may take a variety of forms but, in every instance, will act as a chemical reducing agent in respect to the hexavalent chromium solution. Thus, for example, oxalic acid or titanium shavings may be added to and dissolved in the solution or a titanium or titanium alloy body may be immersed in the solution for a substantial period of time sufficient to result in the reduction of enough chromium ions to the trivalent form to start the coating operation at the desired rate. Alternatively, the initial solution may be composed of a trivalent chromium salt and a suitable acid. Best results may be obtained in accordance with our researches to date through the use of chromium trifluoride and oxalic acid, but we contemplate the use of other trivalent chromium compounds as well as other acids such as hydrofluoric, sulfuric, hydrochloric, fluoboric and trichloroacetic. It is, however, essential that both trivalent chromium and fluoride ions be present in these coating solutions.

We believe that the thickness of the films or coatings formed on the titanium articles treated in accordance with this invention is self-limiting and that only the thinnest type of deposit can be formed, regardless of time or temperature circumstances or reagent concentrations or ratios. Apparently, there can be no deposition on an article. once the titanium surface portion thereof hasbeen covered as a result of chromiumcompound reduction; and, while the addition of more titanium metal or alloy to they chromium compound immersion solution will result in more chomium reduction, any resulting deposition of chromium reduction product will beconfined to the exposed titanium containing surfaces of such added ma terial. A deposit of only a few molecules in thickness is sufficient to arrest deposition on a surface, and we have found that the articles of this invention invariably bear deposits of this magnitude, when suflicient time has been allowed for the system to arrive at an equilibrium condition with respect to the coating reactions.

In its articleaspect, the present invention uniquely combines the strength and weight characteristics of titanium and its alloys with the surface characteristics of I chromium or other metals. Such an article is produced by immersing in an aqueous acid solution of a chromium compound containing trivalent chromium and fluoride ions, a work piece of titanium or titanium alloy and removing the work piece from the solution when a coating has been formed on the said article. However, this invention also includes an article which has a composite coating made by electrodepositing nickel on the aforesaid article and then heating the article and thereby forming a strong alloy bond between the composite coating and the article. Still further, this invention includes those articles made by plating or depositing other metals on these composite coatings.

In practice, as we prefer to carry out our invention, the surface of the titanium or titanium alloy to be plated is st cleaned by suitable conventional means in use in the electroplating art. Thus, grease and oil are removed by vapor degreasing or by immersion in emulsion or alkaline cleaners, as necessary. Relatively heavy scale or oxide surface formations are removed by grinding or brushing or by dilute hydrofluoric acid pickle. After the cleaned article has been rinsed with water, it is immersed in a chromium immersion plating bath selected from among the several composition examples listed below; and, under the conditions therein specified, oxide films are thereby removed from the article, and the way is open for the deposition of a coating of this invention on ill the resulting exposed titanium surface. After such coating has been established, the article is removed from the immersion plating bath and thoroughly rinsed with water and then preferably immediately plated with nickel from a suitable bath, such as a Watts type nickel bath, under standard conditions. We have employed a bath containing 240 gms. per liter of NiSO .7H O, 45 gms. per liter NiCl .6H O, and 30 gms. per liter of H 30 The article is kept in the bath for twentyminutes, while the temperature of the bath is maintained at about 43 C. with the pH between about 4.5 and 5.0 and the cathode density between about 20 and about 65 amps. per square foot. The length of time devoted to this phase of the process of this invention will be determined by the thickness of the plate desired since time bears a direct relationship to plate thickness.

Best results can be obtained in this electroplating operation through the use of relatively low current densities of 10 to 20 amps. per square foot during the first few minutes of plating at which time the current can be increased to conventional plating levels. We contemplate the provision of a chromium plate on top of the resulting nickel plate; and, for this purpose, we prefer to use the electroplating bath disclosed in US. Patent Nos. 1,581,188 and 1,802,463 to Fink. A bath composition of about 250 gms. per liter of chromic anhydride and about 2.5 gms. per liter of sulfuric acid will produce consistently good results Where the bath temperature is about 50 C. and the cathode current density is approximately 150 amps. per square foot.

Where it is desired to improve the degree of adhesion of the resulting composite coating and the base metal body for special uses under severe conditions, the resulting article can be heated to a temperature between about 300 C. and about 600 C., suitably about 400 C., and maintained at that temperature for between about 10 minutes and about 90 minutes, a period of about 30 minutes being preferred when the temperature approximates 400 C. The articles prepared in accordance with this procedure show evidence of interdifiusion and the formation of an alloy bond between the coating or plate and the titanium base article. It will be understood, however, that variations on this procedure may be employed with 1 the expectation of consistently obtaining equally satisfactory results through the inter-relation of the time and temperature of the heat treating step. Generally, the higher the temperature, the shorter the time, and the lower the temperature, the longer time to produce a given bonding effect.

The following examples, of solution compositions for producing uniformly satisfactory immersion chromium films on titanium surfaces are offered not by way of limitation of this disclosure or the appended claims, but merely for the purpose of illustration of this particular aspect of our invention:

a on. Hydroflouric Acid, HF (50%) 60 mL/l 10 to 240 m Temperature 25 20 to 80 C. Time of immersion 7.min l to 10 min.

This solution is treated at the outset with a reducing agent in the form of titanium turnings or oxalic acid for the production of trivalent chromium from the chrom- J ic acid constituent, the latter acting as a reservoir for chromium and being apparently reduced at a rate stoichiometrically equivalent to the removal of chromium from the solution in the form of metallic chrome.

Those skilled in the art will understand that the timetemperature-concentration relationships are Such that the rom , 5- V a higher the temperature orconcentratiom lhemhorter the time required; toiproduceiai'film.

in quantity and stored without, deterioration and used,

'as"requ'ired and 'is easy"to -use'and oifers the advantages of flexibility of the bath of Examplefsincethe same general time-temperature-concentration relationships prevail here as well.

Example III Preferred Range Chromic Trifluoride, CrFs 40 gms./1 5g1ms./l to 50 gms./ Hydrochloric Acid, HCl (36%) .r 40 mls./l 1O inL/l to 100 ml./l. Temperature, 70 C Time of immersion, about 3 min.

Example IV Preferred Range Chromium Trifluoride, CIFB 20 gms./1 gins/1. to 100 gms. Hydrofluoric Acid, HF (50%)----v- 12 m1./l 5 ml./l. to 20 ml./1. Oxalic Acid, HrCzOr 20 gms./l 10 gms./l. to 100 gms. Temperature, Room Time of immersion, about 20 min.

Example V Preferred Range Chromic Acid, OrO; 200 gms./l. 1'0 gins/1. to 1,000

gms. Fluoboric Acid, HBF4 14 mL/l h 6 IuL/l. to 20 ml./I. Oxalic Acid 20 gms./l 5' gms/i/l. to 100 gms. Temperature, 60 C Time of immersion, about 15 min.

Example VI Preferred Range Chromic Chloride, O1C1a.6H20 5 gms./1 0.1 gmlJl. to

gms. Hydrofluoric Acid, HF (50%) 100 ml./l 10 mL/l. to 200 Temperature, Room Time of immersion; about 15 min.

Example VII Preferred Range Ohromio Chloride, OrOladH O 10 gms./l 2gms./l. to gms./ 65 Fluoboric Acid, HBFr 25 ml./l 10 fnL/l. to 50 ml.tl. Temperature, ROOHL 7 Time of immersion, about 20 min.

There must be at least 2.8 grams per liter of fluoride ions, as shown in Example 4, where a minimum amount of 5 ml. per liter of a 50% hydrofluoric acid is added. This acid has a density of 1.198. Therefore, the weight of the hydrofluoric acid is 5 1.198+2. The amount of fluoride ions would be of this amount. This is equal to 2.8 grams per liter of fluoride ions. There must be a minimum amount of'OZOZ gram of -trivalent :chromium ions perliter, as shown 'in -Example i6, where there is 0.1 "-g ram* per liter 6f Cr Cl -i-6H O added '(-mol. 266:38) 0.1 gram wherefore i is the equivalent of 0. 02 gram of trivalentchromium. I

Having thus I described thepresent invention so i that others-skilled F in the f art may be able to understand and practice the same, we stateithat what we desire-to secure by Letters Patent is defined in what isclaime'd.

.Whatzis claimed is: 1. 'A rmethod -of providing 1 a chemical displacement coatingon .an article of aumetalzselected fromtheigroup consisting :of: titanium and titanium-basewalloys which comprisesthe-steps -.of .contacting:the surface of --said article with anelectrolyte' consisting essentially of an aqueous acid..- solution' containing at. least. 0.02 ggram per liter.- of trivalent, chromium ions and; at least. 218 grams .perliter of fluoride ionsratatemperature between about t '20 C. 'and about 80 "for -:betwe enabout one minute and about .30 -minutes, and ,"depositing "chromium by chemical displacement from said solution onto -said;surface.

2. A method of providing a composite coating on an article of a metal selected from the group consisting of titanium and titanium-base alloys which comprises the steps of contacting a cleanedsurface of said article with an electrolyte consisting essentially of anaqueous solution containing at least 0.02 gram per liter of trivalent chromium ions and at least 2.8 grams per liter of fluoride ions at a temperature between about 20 C. and about 80 C. and depositing chromium by chemical displacement from said solution onto said surface, removing said article from contact with said acid solution when said chemical displacement deposit has been established on the article and contacting the article as a cathode with a Watts-type nickel'plating solution at a cathode density between about 10 to about amperes per square foot and forming a nickel plate on said deposit.

3. The method according to claim 2 comprising the step of thereafter heating said article and maintaining the temperature of said article at about 400 C. for about thirty minutes.

4. The method of providing a composite coating on an article of a metal selected from the group consisting of titanium and titanium-base alloys which comprises the steps of contacting a surface of said article with an electrolyte consisting essentially of an aqueous acid solution containing at least 0.02 gram per liter of trivalent chromiurn ions, and at least 2.8 gram per liter of fluoride ions, depositing the chromium from said bath by chemical displacement onto said surface, removing the article from contact with said bath when said chromium has been formed on said'article, and thereafter depositing a coating of nickel on said chromium coating.

5. The method of providing a coating on an article of metal selected from the group consisting of titanium and titanium-base alloys which comprises the steps of immersing a de-greased and de-scaled surface of said article in an electrolyte consisting essentially of an aqueous acid solution containing at least 0.02 gram per liter of trivalent chromium ions, and at least 2.8 grams per liter of fluoride ions, depositing chromium from said solution by chemical displacement onto said surface and removing'the article from contact with said solution when the chromium coating has been fo'rmed on said surface.

6. The method according to claim 5 including the steps of electroplating nickel on said chromium coating and thereafter heating the article and forming a strong alloy bond between the composite coating and the article.

7. The method according to claim 5 including the steps of electroplating nickel on said chromium coating and then electroplating chromium on the nickel plate.

8. The method of providing a coating on an article of a metal selected from the group consisting of titanium and titanium-base alloys which comprises the steps of chromic anhydride and about 6% by volume of 50% hydrofluoric acid for approximately seven minutes at a temperature of about 250 C. and depositing a coating of I chromium by chemical displacement from said solution onto said surface.

9. A method of providing a coating on an article of a metal selected from the group consisting of titanium and titanium-base alloys which comprises the steps of immersing a surface of said article in an electrolyte consisting essentially of an aqueous acid solution containing at least 0.02 gram per liter of trivalent chromium ions, 10 to 1000 grams per liter of chromic acid, 6 ml. to 20 ml. per liter of 50% fluoboric acid solution, and 5 to 100 grams per liter of oxalic acid for between about 10 to 20 minutes at a temperature between about 20 C. and about 60 C. and depositing chromium by chemical displacement from said solution onto said surface.

References Cited in the file of this patent UNITED STATES PATENTS Bennett Mar. 30, Pink etnal. V V V Sept. 26, Prier May 1, Bowden Jan, 22, Brown Aug.,23, Fink et al. May 2, De Long Oct. 7, Bruno May 16, Sonnino Aug. 29, Spruance Aug. 7, Dean July 21, Thirsk Dec. 28, Beach et al. June 21, Smart "2--. Sept. '25, Missel et a1; Mar. 4,

OTHER REFERENCES V Cor- 

2. A METHOD OF PROVIDING A COMPOSITE COATING ON AN ARTICLE OF A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND TITANIUM-BASE ALLOYS WHICH COMPRISES THE STEPS OF CONTACTING A CLEANED SURFACE OF SAID ARTICLE WITH AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING AT LEAST 0.02 GRAMS PER LITER OF TRIVALENT CHROMIUM IONS AND AT LEAST 2.8 GRAMS PER LITER OF FLUORIDE IONS AT A TEMPERATURE BETWEEN ABOUT 20*C. AND ABOUT 80*C. AND DEPOSITING CHROMIUM BY CHEMICAL DISPLACEMENT FROM SAID SOLUTION ONTO SAID SURFACE, REMOVING SAID ARTICLE FROM CONTACT WITH SAID ACID SOLUTION WHEN SAID CHEMICAL DISPLACEMENT DEPOSIT HAS BEEN ESTABLISHED ON THE ARTICLE AND CONTACTING THE ARTICLE AS A CATHODE WITH A WATTS-TYPE NICKEL PLATING SOLUTION AT A CATHODE DENSITY BETWEEN ABOUT 10 TO ABOUT 65 AMPERES PER SQUARE FOOT AND FORMING A NICKEL PLATE ON SAID DEPOSIT. 